1. Field of the Invention
The present invention relates to ring laser gyroscopes, and more particularly to compensation techniques for correcting error factors therein.
2. Description of the Prior Art
Ring laser gyroscopes have been known in the past. Typically, such gyroscopes take the form of a closed resonating cavity into which two oppositely directed beams are inserted. As the cavity is rotated in inertial space the effective lengths of the opposite beam paths are lengthened and shortened. In consequence the resonating frequencies of the two beams become unequal to produce frequency beats indicating the rotation. Ideally, this beat frequency should be linear with the rotation rate of the ring laser, i.e., the ring laser is thus ideally considered to be a linear angular rate measuring instrument.
In the past, most if not all of the work in laser gyroscopes relied on the assumption that the two beam intensities of the instrument are substantially equal. (See, for example, the article by R. L. Fork and, M. A. Pollack appearing in 1965, Physics Review, A139, 1408). More recently, this assumption has received less favor. Specifically, the proposition of equal beam losses has been abandoned and corrections for backscattering differentials has been expressed in terms of intensity--phase interaction called "winking," a term associated with periodic beam intensity peaks occurring close to the null point or the lock-in threshhold. Thus, intensity differentials have been recognized in the past, however, in association with the instrument performance close to its null point.
In this context one should note that the idealized relationships of a laser gyroscope have now been well developed and corrections for some real effects in the instrument are now commonly practiced. These real, physical effects are generally grouped in three error groupings. The first error grouping affects the accuracy of the null point of the ring laser, i.e. the virtual rate output entailed in a stationary laser gyroscope. This error is typically identified as the null shift error or bias.
The second error source, known as the lock-in error, is typically associated with frequency synchronization of the two opposing beams, an effect resulting from the mutual coupling from the opposite traveling waves by scattering energy from one beam onto the direction of the other. This error is analogically similar to the lock-in of a tank circuit when the oscillation of an external injected voltage approaches that of the tank circuit itself. Simply, as the frequency difference between the two oscillations becomes smaller one of the oscillators will lock the other oscillator. It is in this context that most, if not all, the work associated with backscattering and gain/loss ratio has taken place.
The past solution techniques have, in one way or another, corrected the above two errors. For example, the null shift error is taken out by calibration and the lock-in error is often corrected by dither, i.e. swings of the instrument exceeding the lock-in range. Because of these available corrections, little further analytical work has gone on. As a consequence the third source of error, i.e. the error associated with non-linearities in the scale factor of the instrument over the full dynamic range has not been adequately addressed since this error entails an accumulation of a large number of phenomena, and because of the characteristically large dynamic range of the instrument. Thus, the prevailing practice in the past has been directed at the physics of the instrument close to the null point, which has failed to compensate for most scale factor non-linearities.
Accordingly, techniques which conveniently correct the scale factor of a ring laser gyroscope are desired and it is one such technique that is disclosed herein.